optical fiber transmission systems are very widespread today and support very high speed audio and video transmissions. The need for reliable tools that are capable of detecting faults and deterioration of the physical carrier, i.e. of the fiber, is increasingly felt.
Testing apparatuses normally test transmitted data to identify data transmission degradation. Unfortunately, considering that the systems are highly dynamic thus allowing error-free operation even in the presence of considerable line attenuation, damage beyond repair is already in progress when degradation is identified. The importance of testing systems, which are independent from the transmission apparatuses and capable of indicating not only extreme events (such as loss of optical fiber continuity due to breakage or opening of a connector), but also gradual deterioration in fiber efficiency, is evident.
Systems with such characteristics implementing different technical solutions are currently marketed. The most common employ an optical reflectometer implementing OTDR (Optical Time Domain Reflectometry) technology. The system pumps a light signal pulse at a different wavelength from that used for signal transmission so that it can be easily filtered out ahead of the reception apparatuses without interfering with transmission. The light pulse pumped by the reflectometer laser is backscattered on the fiber and returns to the instrument which uses it to trace the optical power of the line according to distance. The smallest line attenuation can be detected by periodically repeating the test on the fiber and comparing the current and the previously recorded or reference traces.
Off-the-shelf systems of this kind are typically designed to work in long-haul networks and are used to test one optical fiber line at a time by means of one or more optical switching devices.
The matter is more complicated when a multi-branch optical network (i.e. a network with several fibers formed through passive optical branch points from a primary line consisting of a single fiber) is to be tested. Analyzing the optical reflectometer trace is more complex because the backscattered light from the various fibers is summed in the branch point before returning to the reflectometer. It is consequently difficult to identify the fiber in the network where the variation may have occurred.
Testing each fiber would obviously increase costs both in terms of passive elements needed to pump optical signals at testing length into each optical fiber (WDM, optical filters, switch ports, etc.) and decreased analysis speed of the entire network.
An OTDR trace analysis method is described in U.S. Pat. No. 6,028,661 dated 22 Feb. 2000. According to the described method, the trace acquired by the reflectometer is analyzed by studying the correlation between adjacent points in the trace employing the solution of a system of equations based on the minimum square method. The solution of this system of equations (the number of which is equal to the number of branches forming the network), is used to estimate an attenuation coefficient for each branch. The variation of one of the coefficients indicates the presence of a variation in the corresponding fiber. This method is rather complicated and consequently slow to run and difficult to implement.